An ancient stone bridge was discovered on the dried up lakebed of Poyang lake in Jiujiang, eastern China in 2013

China has a fifth of the globe’s population but only 7 percent of its available freshwater reserves. Moreover, its water resources are not evenly distributed. The lands north of the Yangtze River—including swaths of the Gobi desert and the grasslands of Inner Mongolia—are the driest, but more than half of China’s people live in the north.

Water is not well managed in China. Nearly two-thirds of water withdrawals in China are for agriculture. Due to the use of uncovered irrigation channels (leading to evaporation) and other outdated techniques, a significant portion of that water never reaches the field.

A new paper by scientists in China, Japan, and the U.S. published in the Proceedings of the National Academy of Sciences sounds the alarm: “China faces … major challenges to sustainable agriculture,” the authors write. Failure to conserve water resources could threaten China’s food security, a longtime priority for the country’s leaders.

When it comes to fresh water, geography did not bless China. “Agriculture is located mainly in the dry north, where irrigation largely relies on groundwater reserves,” the authors write. Meanwhile, due to unsustainable withdrawals, China’s aquifers are fast being depleted.

The paper analyzes water usage for four key crops (rice, wheat, soybeans, and corn) and livestock (poultry, pigs, and cows) in China. Taken together, those make up more than 90 percent of China’s domestic food supply.